1. Field of the Invention
Advantages of Digital CATV Return Path
To offer ever higher speed data services to meet the increased demand, cable operators are rolling out DOCSIS 3.0 services to connect homes and businesses. DOCSIS 3.0 allows upstream and downstream “channel bonding” that enables data connection speeds as high as 100 Mb/s in the downstream direction. Data services to home subscribers have historically been “asymmetrical”, with upstream data speeds lower than downstream data speeds. However, as cable operators serve more and more small businesses, it has become imperative to offer symmetrical data speeds up to and in excess of 100 Mb/s.
DOCSIS 3.0 systems allow cable operators to provide speeds up to 120 Mb/s or up to 240 Mb/s in the upstream path as well, depending on bandwidth allocated to upstream communication, matching the downstream speeds and allowing for the provision of symmetrical (100 Mb/s) data services. This requires a full implementation of DOCSIS 3.0 services in the upstream, including the bonding of four or eight 6.4 MHz channels as well as increasing the modulation level from the previously deployed 16-QAM to 64-QAM. This upgrade allows for higher upstream data speeds but also dramatically increases the performance required of the upstream lasers. Industry also announced further increases in the upstream speed to up to 360 Mb/s and 480 Mb/s. These further increases are allowed by expansion of the upstream bandwidth from 5-45 MHz or 5-65 MH to 5-85 MHz and by increasing modulation levels to 256-QAM. These capacity increases may be supported with any service platform such as DOCSIS platform or any other platform that uses frequency division multiplexed signals.
Cable operators are discovering that many of their older analog return links are unable to support these higher performance requirements. They are increasingly turning to digital upstream links that provide superior signal-to-noise performance, higher dynamic range and a much larger power budget to ensure that the current, increased service deployments and future broadband upgrades with DOCSIS 3.0 or other platform can be supported.
Digital links also do not degrade in quality with increasing link lengths and decreasing RF signal level (for signal levels above the receiver sensitivity) and also simplify the design, alignment and maintenance of the return path of hybrid fiber coax (HFC) networks. Consequently, cable operators are increasingly turning to digital technology for the return path of their HFC or fiber-deep networks.
Advantages of Digital CATV Downstream Path
Currently, very expensive 1550 nm externally modulated (ExMod) lasers are employed to transport downstream analog channels in CATV systems. The use of the 1550 nm wavelength band is so that C-Band EDFAs can be utilized to support analog channel transport over 100 km and longer fiber link distances encountered in broadband HFC networks with consolidated headends. The use of expensive externally modulated lasers is necessitated by the requirement of ultra-low laser chirp in order to avoid dispersion-induced second-order distortion of the analog channels.
Digital downstream links would be advantageous, from performance, reach, and capacity perspectives, over current analog links. However, the large BC bandwidth of anywhere from 50 MHz to 1000 MHz has made it cost prohibitive in the past to use A/D converters to convert the entire forward (downstream) band into a multi-Gb/s digital signal and to transport the resultant data rate in a cost-sensitive access part of the network from headends/hubs to optical nodes.
The current trend towards reduced bandwidths for the analog channel band and larger bandwidths for the narrowcast (NC) band with QAM channels makes the use of digital A/D technology feasible for digitizing the analog downstream path of CAN systems. When analog BC bandwidths are reduced below 300 MHz digital downstream links become superior, from both cost and performance perspectives, over analog links. The parallel trend of falling cost for A/D converters and high data rate transport modules and components also made it feasible to apply the digitization approach to wider bandwidths than 300 MHz.
2. Discussion of the Related Art
Description of Conventional Digital Return CATV Systems
A digital return path includes a digital transmitter (DT), located at a cable “node”, that digitizes the analog cable return path signal (analog signals here is applied to RF carriers frequency division multiplexed into return bandwidths, the carrier are either purely analog or QAM or other RF modulated digital signals); and a digital receiver (DR) that converts this signal back into an analog signal at the cable system hub or headend. A block diagram of a typical DT is shown in FIG. 1.
The cable return path RF signal is first low-pass filtered (LPF) to band-limit the signal and is then amplified (AMP). This signal is then digitized using an A/D converter at a sampling rate determined by a CLOCK signal whose frequency depends on the bandwidth of the cable return signal. This bandwidth differs in different parts of the world (for example, 45 MHz in North America and 65 MHz in Europe) and will also change in the future as MSOs start allocating more bandwidth to the return path. For example, some MSOs are thinking about return bandwidths in the 85 MHz to 200 MHz range.
The parallel data bit streams from the A/D output is then fed into a Processor/Mux unit. The Processor/Mux can be, for example, a microprocessor, a field-programmable gate array (FPGA), or other combination of software and chip-sets. The Processor/Mux unit performs such signal processing functions such as framing, dithering, formatting and encoding. It can also perform remote management and monitoring of the DT.
The Processor/Mux unit of the DT can also perform multiplexing of the digitized input RF signal with a second digital (optical) signal that arrives via the bi-directional optical connector of the DT. In this manner, one set of upstream data signals (e.g., from local small and medium businesses) could be multiplexed with another set of upstream data signals from a different location. Furthermore, several DTs can be optically cascaded in a bus network and the digital signal from the previous DT in this cascade can be multiplexed or combined digitally with the digitized RF signal from this location.
The multiplexed digital signal is then serialized using a serializer/de-serializer (SerDes) and this high-speed digital signal modulates an upstream laser transmitter. As previously mentioned, there is also a photodiode receiver that may be used for detecting an optical input signal that is multiplexed with the RF locally digitized input signal. FIG. 1 illustrates the case where the optical module is a small form-factor pluggable (SFP) component but any other type of bi-directional optical subassembly could be employed in practice.
It is possible to “segment” the node and double the return bandwidth per subscriber by digitizing two input RF signals and multiplexing both of them on the same digital return signal. This method of segmentation (by multiplexing two or more digitized return signals into one data stream) allows for using one wavelength per two or more segments thus preserving fiber capacity. FIG. 2 shows the block diagram of such a dual-channel digital transmitter.
There are now two analog RF input signals on two paths into the DT, two RF filters to band-limit the signals, and two A/D converters to digitize these signals. There is again a Processor/Mux unit that multiplexes these two digital streams (and a third data signal from the optical receiver) into a single digital signal. A SerDes serializes the parallel data bit stream output of the Processor/Mux and this high-speed digital signal modulates the upstream laser transmitter.
As before, there is also a photodiode receiver for detecting an optical input signal (carrying local data from SMBs or data from a previous DT in series with this DT or data from a network control unit) that is multiplexed with the two digitized RF input signals.
The digital return system may include a single DT or an optical cascade of DTs connected to a single-channel or dual-channel digital receiver (DR) over a length of fiber. Since the link is digital, the link length can vary from 0 km to >100 km with little degradation in link performance or output level at the DR output. A block diagram of a typical dual-channel DR is shown in FIG. 3.
The optical input to the DR is detected by a photodiode (or APD) receiver. This serial data stream is de-serialized using a SerDes and fed to a Processor/Demux unit that separates out the two embedded digital return signals from the Management/Monitoring and data signals. The two digital return signals are fed to D/A converters whose sampling rates depend on the bandwidth of the return signals.
The two analog signals are amplified (AMP) and low-pass filtered (LPF) by filters with cutoff frequencies equal to the bandwidth occupied by the return signals. The output levels of the two analog signals are set using Variable Attenuators (Var. Att.) and management software. The output level and signal-to-noise (SNR) ratio of the analog signals do not vary significantly with the fiber span length, an important advantage over analog return systems where both the SNR and output levels of the analog signals degrade rapidly as the fiber span length is increased.
The DTs and DRs described above can be used for forward bandwidth digitization and transport as described above. Hence, the signal digitization and transport in all aspects described above can be used for reverse and forward signal transportation with all the advantages pertinent to the baseband digital signal transmission.